Blender runnning slow in weight paint mode
The lag is probably an indicator that Blender is trying to recalculate and redisplay one or more procedural modifiers while you're trying to weight paint.

Try temporarily disabling any rigs (which would cause Blender to recalculate mesh deformations to match the rig pose), particle emitters (recalculating position of all the emitted children), modifiers etc. that are using the weight paint.

Edit

If it's still lagging, your system may actually be the bottleneck. Many of the processes Blender must go through are single-thread processes that can't be spread out over multiple cores. Since 100% usage of a single thread/core is less than the total capacity of your CPU/GPU, it doesn't display as 100% usage.

You can confirm this by checking the individual core utilization in Task Manager. Look for single cores that are approaching max utilization.

Edit 2

You could also try using local view by pressing / to hide everything else in the scene. 

Your power settings in Win10 could be the issue. Open the start menu, type Edit power plan, go to Change advanced power settings, and check the Processor power management > Maximum processor state.